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Highly Increased Capacitance and Thermal Stability of Anodic Oxide Films on Oxygen-Incorporated Zr-Ti Alloy

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Highly Increased Capacitance and Thermal Stability of Anodic Oxide Films on Oxygen-Incorporated Zr-Ti Alloy Title Highly increased capacitance and thermal stability of anodic oxide films on oxygen-incorporated Zr-Ti alloy Author(s) Habazaki, H.; Kobayashi, K.; Tsuji, E.; Zhu, C.; Aoki, Y.; Nagata, S. Journal of Solid State Electrochemistry, 21(10), 2807-2816 Citation https://doi.org/10.1007/s10008-017-3607-2 Issue Date 2017-10 Doc URL http://hdl.handle.net/2115/71554 Rights "The final publication is available at link.springer.com". Type article (author version) File Information ZrTi_ox_rev.pdf Instructions for use Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP Highly increased capacitance and thermal stability of anodic oxide films on oxygen-incorporated Zr-Ti alloy H. Habazaki1,*, K. Kobayashi1, E. Tsuji1, C. Zhu1, Y. Aoki1, S. Nagata2 1Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan 2Institute for Materials Research, Tohoku University, 2-1-1, katahira, Aoba-ku, Sendai 980-8577, Japan *Corresponding author: Phone & Fax: +81-11-706-6575, e-mail: [email protected] 1 Abstract Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm-3 ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO2 phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m-2, which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO2 phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application. Keywords; anodic oxide, Zr-Ti alloy, dielectric material, anodizing 1. Introduction Anodizing of metals and alloys is of practical importance for many applications, including surface treatments of aluminum and magnesium practical alloys for corrosion protection, wear resistance and surface decoration [1-3], production of Al and Ta electrolytic capacitors [4-8] and fabrication of nanomaterials for functional devices [9-11]. Both compact, so-called barrier-type anodic films and porous-type anodic films are formed on a range of metals, depending upon anodizing conditions. The most important practical application of the barrier-type anodic films is the use as a dielectric layer in electrolytic capacitors. Ta capacitors consist of porous Ta anode sintered from Ta powder, a thin dielectric layer formed by anodizing of the Ta anode and conducting polymer or MnO2 2 cathode. Ta capacitors possess superior properties, such as high reliability, high volume efficiency and low equivalent series resistance [7]. However, because of limited natural resources of Ta and relatively high cost of Ta raw material, there is a continuous demand of the development of alternative materials for electrolytic capacitor. Niobium is a candidate since its physical and chemical properties are similar to those of Ta and the anodizing behavior of these two metals is also similar to each other. It is, however, often claimed that the reliability of niobium capacitor is lower than that of Ta one, because of higher susceptibility of field crystallization of anodic niobium oxide [12-18], which increases dielectric loss of the capacitor, and large bias potential dependence of the capacitance originating from the n-type semiconductor character of the niobium oxide [19]. Alloying of niobium to form a non-equilibrium, uniform solid solution is effective in suppressing the field crystallization and avoiding the degradation of the dielectric properties [14,15,20-23], although the non-equilibrium solid-solution alloys are not suitable for application to the current capacitor technology, which includes high temperature sintering of the metal powders. Ti is also an abundant element and forms an anodic oxide with relatively high permittivity, but shows an amorphous-to-crystalline transition at low formation voltages less than 10 V. The transition induces gas generation on the crystalline oxide, making the anodic films highly defective [24-26]. Again, alloying of Ti is effective in suppressing the amorphous-to-crystalline transition, and the amorphous anodic films formed on the solid-solution Ti alloys are promising as dielectric films for capacitor application [27-30]. These anodic oxide films are also promising for a hybrid inorganic-organic field effect transistor [31,32]. However, Ti shows limited solubility to many alloying elements including silicon, tungsten, molybdenum and aluminum at equilibrium. It is not easy to form solid solution Ti alloy anode by sintering of the alloy powders for capacitor application. A Ti-Zr alloy system is interesting because of the formation of solid solution in a wide composition range at equilibrium. In addition, it was reported that the capacitances of the anodic films formed on the Ti-Zr alloys were markedly enhanced at the Zr-rich alloy composition, where 3 nanocrystalline ZrO2 phase precipitated in the amorphous oxide matrix [33]. The capacitance of the anodic films on the Zr-rich Zr-Ti alloy was higher than that on Ta and the band gap of 3.65 for the anodic oxide film formed on Zr-20 at% Ti alloy was larger than that on TiO2 (3.1-3.2) [34]. Such Zr-Ti alloy may become a promising novel material for the electrolytic capacitor application. For practical application, it is important to investigate the thermal stability of the anodized Zr-Ti alloy, particularly since both Zr and Ti are highly reactive with oxygen and the solubility of oxygen to both metals are relatively high. In this study, we investigated the change in the thickness, structure and dielectric properties of the anodic films on a Zr-Ti alloy during heat treatment at 473 K in air. We also prepared the anodic film on an oxygen-incorporated Zr-Ti alloy to examine the influence of incorporated oxygen in alloy on the thermal degradation of the anodized alloy. Here, we report marked improvement of the thermal stability and further increase in the capacitance of the anodic film by the oxygen incorporation in alloy. 2. Experimental The Zr-Ti alloy containing 24 at% Ti was prepared by magnetron sputtering (Shimadzu, SP-2C system) on glass or silicon wafer substrate. The glass substrate of 1 mm thickness was degreased in an aqueous solution containing 30 g dm-3 surfactant (Okuno Chemical, Top Alclean 30) at 333 K and the silicon wafer substrate was thermally oxidized at 1173 K in air to form a thermal oxide film. The target consisted of 99.9% Ti disc of 100 mm in diameter and 6 pieces of 99.9% Zr discs of 20 mm in diameter; the latter discs were placed symmetrically on the sputter erosion region of the Ti disc target. After installing the substrates and targets in the sputtering chamber, the chamber was evacuated to less than 5 × 10-5 Pa and then, alloy deposition was conducted under ~0.3 Pa argon at 0.5 A for 30 min. In order to get the deposited alloy films of uniform composition and thickness, the substrate holders were rotated around the central axis of the chamber as well as their own axes. The composition of the alloy film was determined by Rutherford backscattering spectroscopy and the thickness of the deposited film, determined by cross-sectional SEM observation was ~490 nm. 4 The incorporation of oxygen in the deposited alloy was performed by anodizing and subsequent heat treatment in vacuum. The deposited Zr-Ti alloy was anodized to several formation voltages at a constant current density of 50 A m-2 in 0.01 mol dm-3 ammonium pentaborate electrolyte at 293 K. Then, the anodized specimen was heat-treated at 823 K for 1 h in vacuum at <10-3 Pa. The dielectric oxide films were finally formed by anodizing of the as-deposited and oxygen-incorporated Zr-Ti alloy specimens at a constant current density of 50 A m-2 to 50 V with current decay for 30 min in 0.01 mol dm-3 ammonium pentaborate electrolyte at 293 K. All anodizing experiments were performed using a two-electrode cell and a platinum sheet was used as a counter electrode. Thermal degradation of the anodized specimens were examined by heating at 473 K in air for various periods of time. Dielectric properties of the anodic oxide films were examined by an electrochemical impedance spectroscopy (EIS). The measurements were conducted in 0.1 mol dm-3 ammonium pentaborate electrolyte by applying 50 mV (rms) sinusoidal alternating voltage at an open circuit potential using a NF Block, 5020 frequency analyzer combined with a Hokuto Denko, HA-500 potentiostat. The obtained spectra were fitted using a Zsimpwin software. Phases in the specimens were identified by a glazing incidence X-ray diffraction (GI-XRD) method. A Rigaku, RINT-2200 system with Cu Kα radiation (λ = 0.15418 nm) was used and the incident angle, α, was set to 1°. Surfaces of the specimens were observed by a JEOL, JSM-6500F field emission scanning electron microscope operated at 10 kV, while cross-sections of some specimens were observed by a JEOL, JEM-2000FX transmission electron microscope operated at 200 kV. Electron-transparent sections were prepared by a Hitachi, FB-2100 focused ion beam system.
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